台灣西南海域陸源有機碳沉降受鄰近島嶼型河川顆粒傳輸影響之研究

Abstract

本研究藉由有機碳δ13C值及有機碳/氮莫耳比值作為陸海源示蹤劑，估算陸源有機碳堆積通量，以釐清陸源有機碳經由島嶼型小河川(曾文溪、二仁溪及高屏溪)輸入台灣西南大陸邊緣海域後的傳輸路徑與堆積埋藏之最終宿命。 研究結果顯示台灣西南海域表層沉積物有機碳、有機氮的空間分佈明顯受到曾文溪、二仁溪及高屏溪輸出之陸源顆粒的影響。表層沉積物有機碳、有機氮含量的空間分佈變化呈現由北往南降低的趨勢，且明顯受到沉積物顆粒大小所控制。表層沉積物中有機碳δ13C值大致呈現由近岸往外海變重的趨勢，有機碳/氮莫耳比值亦呈現相似的變化趨勢。近岸陸棚沉積物中有機碳來源主要以陸源為主，特別在曾文溪與高屏溪河口、高屏峽谷內及峽谷中段的兩側陸坡區域可發現較高比例之陸源有機碳的堆積。外海陸坡區域沉積物中有機碳來源則以海源為主。澎湖群島南側陸棚沉積物亦明顯以海源有機碳為主。高雄港北側陸棚沉積物則可能受到污水排放中所含的有機污染物之影響，有機碳δ13C值呈現明顯陸源的特徵。 台灣西南海域的陸源有機碳堆積效益相當低，大部分河川輸出的陸源有機碳並未堆積在本研究區域之陸棚及陸坡內。曾文溪輸出的陸源有機碳只有少部份堆積在河口外圍，而大部份可能被海流帶往其他區域沉積。高屏溪輸出的陸源有機碳則大部份可能直接經由高屏峽谷往深海(南海)傳輸埋藏。 台灣島嶼型小河川輸出之陸源物質並未像其他大陸型大河之輸出多堆積於河口三角洲或近岸陸棚，台灣島嶼型小河川輸出之陸源物質可經由峽谷地形往深海傳輸與埋藏。峽谷對於島嶼型小河川陸源物質往深海之傳輸扮演著相當重要的角色。

Huge amount of suspended materials together with organic carbon exported from Kaoping River, Tsengwen River and Erjen River could serve as a major carbon source and may influence biogeochemical carbon cycle in offshore area, southwest of Taiwan. However, little is know concerning fate of this terrestrial organic carbon. This study aims at understanding fate of this terrestrial organic carbon exported to the continental margin, offshore southwestern Taiwan through investigating spatial variations of organic carbon, organic nitrogen, grain size, and organic carbon isotopic compositions in surface sediments to evaluate net terrestrial organic carbon burial flux in the narrow continental shelf settings. Results show that surface sediments in offshore area, southwest of Taiwan were strongly influenced by terrigenous materials exported from Kaoping River, Tsengwen River and Erjen River. Organic carbon and nitrogen content in surface sediments increased from offshore Penghu island to offshore Kaoping River. Organic carbon and nitrogen content showed strong positive linear relationships with mud content, which indicates that grain size is one important factor in controlling distribution of organic carbon and nitrogen in this study region. The appearance of lighter organic carbon near Kaoping River, Erjen River and Tsengwen River mouth and gradually shifting from lighter at shelf to heavier at slope demonstrated that organic carbon are shifting from terrigenous domination to predominant marine away from the shore. Comparing to those predominantly terrigenous sediments near rivers , sediments offshore Penghu Island are characterized by mostly coarse-grained and biogenic in origin. Offshore Tsengwen and Erjen River are mostly silty to coarse-grained sediments with a major fraction composed of terrigenous sediments. Offshore Kaoping River were dominated by terrigenous fine-grained muddy sediments. In addition, patches of higher organic carbon with lighter isotopic compositions were found offshore north of the Kaoshiung Harbor, probably a result of organic pollutant from sewage outlets. Terrestrial organic carbon burial flux in our study area is 0.063 Mt/yr, which is only 12 % of the total particle organic carbon exported from Kaoping river, Erjen River and Tsengwen River. Higher fraction(>90%) of terrestrial organic carbon occurred in the lower Koaping Canyon indicates that a very important part of the terrestrial organic carbon exported from Kaoping River may transport directly into deep ocean through the Kaoping Canyon. Kaoping Canyon is playing a significant role in transportation and burial of terrestrial organic materials from the Kaoping River to deep ocean.